U.S. Pat. No. 4,975,636 to Desautels for a "Method and Apparatus for Selecting and Displaying a High Resolution Window from a Main Display", hereby incorporated by reference, describes an oscilloscope display with a portion of a main trace shown in a window with a different scale. The Background of the Invention of this patent provides a good explanation of why the operator of a digital oscilloscope finds it useful to be able to examine a portion of a main trace more closely using a second trace with higher resolution. To provide this feature, the '636 patent describes how a software program operates in conjunction with a single set of timebase hardware to allow the operator to concurrently select both a main trace and an second trace with a higher resolution than the main trace showing a subsection of the main trace in more detail. As is described in column 3 of this patent, data is acquired with relatively high horizontal (time) resolution that is more than is needed for a typical main display. However, this extra horizontal resolution is used to produce the second trace with higher resolution. The operator identifies the region to be examined in detail by positioning a pair of vertical cursors at the beginning and end of it on the main trace, and this portion of the main trace is then shown in more detail in the second, higher resolution trace.
U.S. Pat. No. 5,039,937 to Mandt et al. for a "Method and Apparatus for Providing Compressed and Expanded Displays on a Digital Oscilloscope", hereby incorporated by reference, describes another approach to giving a digital oscilloscope operator a detailed view of part of an acquired waveform and another view showing the relationship of that detailed view to a bigger picture of the acquired waveform. In this approach, the overview display is relatively small and is called a "scroll bar waveform". The scroll bar waveform contains a highly compressed and simplified version of the waveform data, and a pair of brackets associated with it. The location of these brackets defines the amount of the scroll bar waveform data that is shown in a larger and more detailed main display that generally has higher resolution.
U.S. Pat. No. 5,129,722 to Mader et al. for an "Expansion Windowing System for a Measurement Test Instrument", hereby incorporated by reference, describes a system for expanding a waveform that includes an expansion viewport and a cursor. The expansion viewport has a centerpoint whose location is controlled by where the horizontal cursor intersects the waveform, and has horizontal and vertical dimensions that are controlled by two knobs on the front panel. Activating an expand switch causes the display to be filled with the contents of the expansion viewport.
Japanese Laid Open Patent Application No. 4-198620 filed Jul. 24, 1992 by Yokogawa Electric Corporation for a "Waveform Analyzer" and published on Feb. 18, 1994, describes a display system having an enlargement region of a main trace identified with a box-type region. Statistical waveform analysis techniques may be applied to the whole waveform or to only that portion of it that is within the box-type region.
In prior art oscilloscopes presently built by Applicant's corporation, Tektronix, Inc., a software routine performs interpolation or decimation on acquired waveform data as required to convert it into trace data. Acquired waveform data describes the voltage levels present on an input signal at regular time intervals. This data is stored at addresses that correspond to successive acquisition times, so that there is a linear relationship between successive address numbers and successive data acquisition times (ignoring the fact that physical addresses may be in a circular memory or that data may be demultiplexed to multiple memories because of speed considerations). The successive time intervals between samples in the acquired waveform data may be too long or too short relative to the detail with which the operator wants to examine the data. For example, the data record may be 50,000 points long, while the screen is only capable of displaying 500 points. Therefore, to view the whole acquisition in one display, a 1:100 decimation is required. Conversely, when a user wants a more detailed view of a portion of a waveform and the original data acquisition acquired fewer valid data values in the region of interest than would be required to produce such a display, interpolation (e.g., linear or sin(x)/x) is performed to supply the additional points for that display. The software that performs this function, the I/D (interpolation/decimation) routine, is now well known, having now been part of oscilloscope products for several years.
In more than one oscilloscope on the market today, including the 11000 Series Digitizing Oscilloscopes, the operator is provided with up to three inter-related displays of an acquired waveform, a main trace and two zoomed traces. The two zoomed traces provide magnified displays of highlighted (i.e., intensified) portions of the main trace. Each zoomed trace is an expanded region of the main trace that is displayed in its own window. Each window may be controlled independently to have different horizontal and vertical settings. Having independent settings necessitates having separate graticules or separate scale indications for each trace on a common graticule.
While this independence of the two expansion windows is convenient under some circumstances, it is also extremely inconvenient under other circumstances. For example, in comparing the details of two pulses it is most convenient to overlay them and see where they coincide and where they diverge. However, it is also frequently necessary to change the horizontal (time) and vertical (amplitude) expansion factors to look more closely at the waveforms during the course of the comparison. With independent expansion windows this means replicating the setting adjustments for the two displays and switching control functions between the different zoomed displays in order to accomplish this. Thus, using a prior art dual zoom oscilloscope for some operations is inconveniently complicated and confusing.